Map of dietary energy availability per person per day in 1961 (kcal/person/day).

Map of dietary energy availability per person per day in 1979-1981 (kcal/person/day).[1]

Map of dietary energy availability per person per day in 2001–2003 (kcal/person/day).[1]

no data

<1600

1600-1800

1800-2000

2000-2200

2200-2400

2400-2600

2600-2800

2800-3000

3000-3200

3200-3400

3400-3600

>3600

Diet plays an important role in the genesis of obesity. Personal choices, advertising, social customs and cultural influences, as well as food availability and pricing all play a role in determining what and how much an individual eats.

The dietary energy supply is the food available for human consumption, usually expressed in kilocalories per person per day. It gives an overestimate of the total amount of food consumed as it reflects both food consumed and food wasted. The per capita dietary energy supply varies markedly between different regions and countries. It has also changed significantly over time.[2] From the early 1970s to the late 1990s, the average calories available per person per day (the amount of food bought) has increased in all part of the world except Eastern Europe and parts of Africa. The United States had the highest availability with 3654 kilo calories per person in 1996.[3] This increased further in 2002 to 3770.[4] During the late 1990s, Europeans had 3394 kilo calories per person, in the developing areas of Asia there were 2648 kilo calories per person, and in sub-Sahara Africa people had 2176 kilo calories per person.[3][5]

Change over time of the macronutrient composition of the US male diet.

Change over time of the macronutrient composition of the US female diet.

USDA chart showing the increase in soda consumption and the decrease in milk consumption from 1947 to 2001.[6]

From 1971 – 2000, the average daily number of calories which women consumed in the United States increased by 335 calories per day (1542 calories in 1971 and 1877 calories in 2000). For men, the average increase was 168 calories per day (2450 calories in 1971 and 2618 calories in 2000). Most of these extra calories came from an increase in carbohydrate consumption, though there was also an increase in fat consumption over the same time period.[7] The increase in caloric consumption is attributed primarily to the "consumption of food away from home; increased energy consumption from salty snacks, soft drinks, and pizza; and increased portion sizes".[7] Other sources note that the consumption of soft drinks and other sweetened beverages now accounts for almost 25 percent of daily calories in young adults in America.[8] As these estimates are based on a person's recall, they may underestimate the amount of calories actually consumed.[7]

As societies become increasingly reliant on energy-dense fast-food meals, the association between fast food consumption and obesity becomes more concerning.[9] In the United States consumption of fast food meal has tripled and calorie intake from fast food has quadrupled between 1977 and 1995.[10] Consumption of sweetened drinks is also believed to be a major contributor to the rising rates of obesity.[11][12]

The portion size of many prepackage and restaurant foods has increased in both the United States and Denmark since the 1970s.[7]
Fast food servings, for example, are 2 to 5 times larger than they were in the 1980s. Evidence has shown that larger portions of energy-dense foods lead to greater energy intake and thus to greater rates of obesity.[13][14]

A 2010 study published in the American Journal of Clinical Nutrition closely tracked 373,803 people over a period of 8 years across 10 countries. At its conclusion, the study reported that meat consumption is positively associated with weight gain in men and women.[15] In response, the National Cattlemen's Beef Association countered that increased meat consumption may not be associated with fat gain.[16] However, a subsequent response controlled for just abdominal fat across a sample of 91,214 people found that even when controlling for calories and lifestyle factors, meat consumption is linked with obesity.[17] Further population studies, reviews, and meta-analysis studies have corroborated the claim that greater meat consumption is linked to greater rates of obesity.[18][19]

Existing evidence from large-scale cross-sectional and prospective cohort studies supports that consumption of sugar-sweetened beverages is correlated with the childhood and adult obesity epidemic in the United States population (Malik et al. 2006). Sweetened drinks containing either sucrose alone or sucrose in combination with fructose appear to lead to weight gain due to increase energy intake.[20] Fructose might also be preferentially metabolized into fat as feedback inhibition as with glucose is missing.[21] In fact, about half of total added sugar consumed in the United States is in liquid form.[22] Teenage boys showed the greatest consumption of sugar-sweetened beverages, upwards of 357 calories per day.[22] In addition to the evidence that humans have a natural propensity towards sugar,[23] there exists additional biological plausibility that explains the correlation of sugar-sweetened beverage consumption and obesity.[24]

A study by the National Center for Health Statistics showed that non-Hispanic black men consumed a larger percentage of their total calories from added sugars than non-Hispanic white and Mexican-American men. Non-Hispanic black men consumed 14.5% of their calories from added sugars compared with 12.8% for non-Hispanic white men and 12.9% for Mexican-American men. Non-Hispanic black women also consumed a larger percentage of their total calories from added sugars than non-Hispanic white and Mexican-American women. Non-Hispanic black women consumed 15.2% of their calories from added sugars compared with 13.2% for non-Hispanic white women and 12.6% for Mexican-American women. No significant differences in percentage of calories from added sugars were found between non-Hispanic white and Mexican-American men or women.[25]

Sugar-sweetened beverages raise concern because they are calorie-dense and yet produce low satiety.[26] There exists a strong correlation between the consumption of liquid calories and total energy intake. Individuals do not tend to decrease solid calories in compensation for increased liquid calories.[24] For example, if there were no compensation for liquid calories, the 40-50g of sugar in each 12 oz. can of soda drunk on a daily basis could lead to a 15-pound weight gain per year.[24] On the national scale, the American Heart Association estimates that approximately half of the total caloric intake increase over the past 30 years is attributable to liquid calories.[27] The high glycemic load of these beverages is also thought to be a contributor to chronic disease, including diabetes and nonalcoholic fatty liver disease.[26][28] Furthermore, there is emerging evidence that suggests sugar may be an addictive substance, further potentiating any individuals’ existing problem of excessive consumption.[23]

The soda industry budget for advertising in 2000 totaled over $700 million in the US, an increase of over $381 million since 1986.[22] Furthermore, standard serving sizes have increased from 8-ounce to 12-ounce bottles between 1977-1996.[22] Price incentives from beverage companies have kept their product prices low.[29]

New agricultural technologies have led to an overall reduction in the cost of food relative to household income, especially in high-income countries. In his popular book, "The Omnivore's Dilemma," the journalist Michael Pollan linked the subsidies offered to farmers of corn, soy, wheat, and rice through the U.S. farm bill to over-consumption of calories derived from these crops and to rising obesity rates.[30] While increased consumption of foods derived from these commodities is correlated with an increase in BMI (at the population level), no current research supports a causal relationship between farm subsidies and obesity.[31] From a policy perspective, the cost of sugar would actually decrease in the US if the commodity support programs in the farm bill were removed, largely due to the tariffs in the farm bill that restrict the importation of lower-cost sugar available on the global market.[32]

Evidence does not support the commonly expressed view that some obese people eat little yet gain weight due to a slow metabolism. On average obese people have a greater energy expenditure than normal weight or thin people and actually have higher BMRs.[34][35] This is because it takes more energy to maintain an increased body mass.[36] Obese people also underreport how much food they consume compared to those of normal weight.[37] Tests of human subjects carried out in a calorimeter support this conclusion.[38]